/*
 * Created by Dev c++ 5.11
 * Author: liyih
 * Date: 2018/03/14
 */

#include <iostream>
#include <list>
using namespace std;
int main()
{
	int n, t, i, len;
	cin>>n;
	len = n;
	while(n--){
		cin>>t;
	}
	cin>>i;
	if(i >=1 && i <= len)cout<<"ok";
	else cout<<"error";
	return 0;
}
